California themed coloring books and crayons for the little ones (for the "boring" car drives).
A few "celeb" magazines for the teens for the car drives or hotel room. (ya, they can get those at home but it can be a special treat)
A big floppy hat for the mom and maybe a baseball style hat for the dad (please, NO KINGS stuff!
)
Something from In n Out (or directions to one, if they eat meat)